The present invention relates to an electrophotographic photosensitive body, an intermediate transfer medium, and an electrophotographic apparatus.
In wet electrophotographic technology, a liquid developing agent formed by dispersing toner in a petroleum solvent is used, and the developing process uses the electrophoresis of toner particles in this petroleum solvent. This wet electrophotographic technology has various advantages that are unrealizable by dry electrophotographic technology. So, the merits of wet electrophotographic technology are being reconsidered recently.
For example, wet electrophotographic technology can realize high image quality because it can use very fine toner particles on submicron order. Also, since satisfactory image density can be obtained with a small amount of toner, this technology is economical and can achieve texture comparable with printing. Furthermore, energy saving is possible because toner can be fixed to a paper sheet at relatively low temperatures.
In electrophotographic technology, transfer efficiency has very large influence on image quality. For example, if a transfer efficiency of 100% is not achieved, i.e., if toner is not entirely transferred onto a paper sheet, the image density lowers, or the image quality lowers in the form of an image blur or the like. Accordingly, it is being desired to realize sufficiently high transfer efficiency, i.e., transfer efficiency close to 100%.
In wet electrophotographic technology, however, toner is fine and a developing agent contains a solvent, so the adhesion of the toner to a photosensitive body is excessively strong. Therefore, satisfactory transfer efficiency is not always obtained in the conventional wet electrophotographic technology.
For example, U.S. Pat. Nos. 5,148,222, 5,166,734, and 5,208,637 have disclosed an electric field transfer method by which toner is transferred from a photosensitive body to a transfer roller by using electric field, and this toner on the transfer roller is then transferred onto a paper sheet by using pressure or the like. In this method, the movement of toner particles from the photosensitive body to the transfer roller is primarily brought about by the electrophoresis of the toner particles in a liquid developing agent interposed between the photosensitive body and the transfer roller. Hence, if the adhesion of the toner to the photosensitive body is excessively strong, an extremely large potential difference must be produced between the photosensitive body and the transfer roller. Unfortunately, no such large potential difference is normally applied. So, sufficiently high transfer efficiency is difficult to achieve when this electric field transfer method is employed.
Jpn. Pat. Appln. KOKOKU Publication No. 46-41679 and Jpn. Pat. Appln. KOKAI Publication No. 62-280882 have disclosed a so-called offset transfer method which transfers toner from a photosensitive body to a transfer roller and from the transfer roller onto a paper sheet by using heat or pressure. This offset transfer method can realize higher transfer efficiency than in the electric field transfer method. However, even this offset transfer method hardly achieves transfer efficiency close to 100%.
As described above, in the wet electrophotographic technology it is very difficult to realize transfer efficiency close to 100% only by improving the transfer method. To improve the transfer efficiency, therefore, a method is proposed by which the surface of a photosensitive body is coated with silicone resin or fluororesin to decrease the adhesion between the photosensitive body surface and toner.
This method can actually improve the transfer efficiency. However, this effect is obtained only in the initial stages. That is, even when a thin film is formed on the surface of a photosensitive body by using silicone resin or fluororesin, high transfer efficiency cannot be maintained for long time periods. The reasons will be described below.
A thin film formed on the surface of a photosensitive body has influences on the electrostatic property of the photosensitive body and on the electrostatic interaction between the photosensitive body and toner. Therefore, to obtain high image quality, this thin film must be formed to be very thin. Unfortunately, a thin film formed by using silicone resin or fluorine resin has low mechanical strength. Hence, when transfer steps are repeated, the surface of this thin film wears and the transfer efficiency gradually lowers.
Additionally, toner remaining on the photosensitive body surface without being transferred onto a paper sheet must be removed by a cleaner. If, however, it is obvious that the transfer efficiency lowers, a stronger cleaner must be used. Since the photosensitive body surface is more or less damaged by a cleaner, this damage to the photosensitive body surface increases if a stronger cleaner is used.
For these reasons, when a thin film is formed on the surface of a photosensitive body by using silicone resin or fluorine resin, the wear of this thin film progresses very rapidly. So, no high transfer efficiency can be maintained for long time periods. Therefore, a thin film formed on the photosensitive body surface is being desired to be able to well decrease the adhesion of toner to the photosensitive body surface and have satisfactory mechanical strength.
Note that the aforementioned problems are described primarily in relation to wet electrophotographic technology. However, such problems are similarly encountered in dry electrophotographic technology, as well as in wet electrophotographic technology.